1. Field of the Invention
The present invention relates to the use of layers or coatings on tubes and threaded joints for tubes, particularly for protecting them from corrosion or for improving the fluid circulation conditions in the interior thereof.
The invention further relates to the tubes and threaded joints for metal tubes which are provided with such an internal coating or layer. Such tubes and threaded joints are used for exploiting deposits of oil and gas, in particular for wells into which injections of water are effected.
The invention further relates to any tube and threaded joint associated therewith, which because of the conditions of use are subjected to severe corrosion by virtue of contact with the fluid flowing therein. This is particularly, and in a non-limiting fashion, the situation with tubes and threaded joints used for circulating water as in, e.g., oil wells or geothermal wells, due to the composition and temperature of the water in contact with the tubes.
2. Discussion of the Background
The internal walls of tubes and the components of joints associated with those tubes may be protected from corrosion by a thin adhesive layer of a plastic coating such as a resin based, e.g., on a polyepoxide. Such a coating has excellent adhesion to steel and very good resistance to corrosion. Irrespective of such corrosion problems, such coatings are also used on tubes and the components of joints associated with those tubes to reduce pressure drops, in particular to reduce the roughness of the surface of the internal wall of the tubes and the components of the joints, and to eliminate to the maximum degree the turbulence phenomena which may occur at the internal walls of the tubes and joints when the fluids are flowing therethrough.
However, at the location of a joint between, e.g., the annular surface of the male end of a tube portion and an annular surface forming an abutment of the female component of the joint, it is found that when the joint is screwed together, the tightening pressure tends to cause the protective coating to crack and split in the immediate vicinity of the contact zone. This results in flaking and very substantial localized corrosion of the subjacent metal thus exposed; in the situation where the coatings are used to improve the surface quality of the walls in contact with the flow of fluid, this results in disturbances in the flow of fluid.
Even if the corrosion involved does not cause immediate leaks, it prevents re-use without repair of the components of the joint after it has been unscrewed. That corrosion may even result in destruction of the abutment zones and the sealing surfaces of the joints.
Different solutions for preventing such localized corrosion have been proposed. They involve sealing rings of elastomers or of certain plastic materials.
Some sealing rings, such as those made of polytetrafluoroethylene (PTFE), have the combined advantages of good mechanical strength and excellent resistance to corrosion and good compatibility with the plastic coatings used for metal tubes and their means of assembly. The rings used may be reinforced by glass or carbon fibers.
A number of examples describe the use of such rings.
Thus, U.S. Pat. No. 3,100,656 describes an assembly of integral type in which the threaded male end of a tube portion is fitted by screwing into a female housing of another tube portion. The internal walls of the tubes and the components of the joint are coated with a layer of a phenolic resin such as phenol formaldehyde, measuring from 0.002 to 0.006 inch (from 0.05 to 0.15 mm). A sealing ring based on polytetrafluoroethylene is partially engaged into an annular housing in the female wall bearing against a shoulder thereof. The ring is of an I-shaped section such that its enlarged external zone engages into the housing while the end of the male element, at the end of the screwing operation, tightens a lateral edge of the ring against the shoulder. The degree of tightening is limited by abutments.
U.S. Pat. No. 4,875,713 describes another method of assembling two tube portions. This involves a sleeve permitting end-to-end assembly of the male ends of two tube portions. The internal wall of the tube is protected from corrosion by a plastic coating formed by a layer of phenolic resin, or epoxyphenolic resin, or by epoxypolyamides. The ring is mounted at the middle of the sleeve in a double groove. The ring is a ring of PTFE reinforced by 5% of glass fibers and 5% of carbon fibers; it is of a reversed U-shape, with the two arms of the U being engaged into two parallel grooves. The two front ends of the male elements partially cover over the top of each arm of the U-shape, thus blocking the joint in its housing. Abutment shoulders at the ends of the sleeve control the degree of screwing of each element.
U.S. Pat. No. 5,236,230 describes an assembly similar to that of U.S. Pat. No. 4,875,713. However, U.S. Pat. No. 5,236,230 uses a ring of PTFE whose section is substantially rectangular, and whose radial height is about 1.5 times the width as measured parallel to the axis. As shown in FIGS. 4 A and 4 B, when the opposite front male ends are clamped against the ring, its initially rectangular section is deformed into a T-shape and its internal surface defines a flow section equal to the flow section of the tubes, which prevents any disturbance in the flow of fluid. The clamping effect is controlled by stop shoulders. There is no disclosure regarding the risks of cracking of the coating at the right angles, or the nearly right angles, which are shown in FIG. 4 B.
Although those different methods of protecting the junction zone of such joints from corrosion have certain advantages, they all suffer from the disadvantage of using rings of elastomer or plastomer which in themselves involve significant problems. Those rings generally have to be set in place with precision in order for them to bear correctly against the sealing surfaces; at each unscrewing and re-screwing operation, they must also be carefully checked and in most cases replaced, because of ageing or plastic deformation. In the case of the relatively rigid rings, such as those of PTFE, they can be severed if they are badly positioned and they may also give rise to cracks in the plastic coating, which is much softer. The rings can thus be expelled from their housing and can be disposed in the interior of the tube, or may be torn away or damaged when the tubes are subjected to internal checking. It is also noted that, at operating sites, e.g., production facilities, such rings necessitate additional handling operations which must be carried out with precision.